It is well known in the electronics industry that static electricity can cause severe damage to IC devices. The generation of the electrostatic charge results from the transfer of electrons within a body (polarization) or the transfer of electrons from one body to another (conductive charging), and is usually due to the interaction of bodies. The magnitude of the charge is primarily dependent upon the size, shape, composition and electrical properties of the substances which make up the bodies. The prime sources of electrostatic charge encountered in a manufacturing facility are essentially the interaction of personnel with insulators. Typically, walking across a carpet, or over a vinyl floor, or the handling, rubbing or separating various materials generates electrostatic charges which are transmitted to the person and causes that person to be charged. When the charged person handles or comes close to a susceptible electronic part, that part can be damaged by direct discharge from the contact or by being subjected to the electrostatic field surrounding the charged person. The generation of 15,000 volts between unprotected workers and electronic parts is not unusual in a typical manufacturing facility.
In general, voltage sensitive parts fail due to dielectric breakdown of insulating layers. In a transistor (MOSFET or TFT) dielectric breakdown usually occurs across the gate insulator. As feature sizes are reduced, in order to increase the number of devices upon a single substrate, this scaling makes the transistor more vulnerable to damage by smaller electrostatic discharges. These discharges may cause catastrophic failure or may shift the transistor operating characteristics out of specification. Failure results when the dielectric strength is exceeded, causing insulator punchthrough and leaving a low resistance short. The high potential difference across the gate dielectric, caused by the presence of static charge, results in charges being trapped in the dielectric and shielding the correct operational electric field from the gate.
Numerous electrostatic discharge protection circuits have been developed to protect the transistor gate dielectric against these high voltage effects. These circuits usually are connected directly to the input contact pads and serve to shunt away or reduce high voltage pulses to a value below the critically disruptive value, while not interfering with device electrical performance. Usually these protection networks include diodes and other elements for which IC chip real estate must be allocated.
It is one object of the present invention to provide a simple, low cost, electrostatic discharge protection network for large area thin film transducer arrays wherein each transducer has associated therewith an addressing circuit including at least one thin film transistor.
It is a further object of the present invention to provide paths for potential equalization of all elements on the array in response to spurious electrostatic discharges.